This invention relates in general to the art of coating and in particular to an improved method for carrying out a process of coating in which one or more layers of coating composition are applied to the surface of an object by advancing the object through a coating zone in which a flow of coating composition is applied thereto, for example, a process of bead coating or a process of curtain coating. More specifically, this invention relates to an improved coating method in the manufacturing of a photographic film, photographic printing paper, a magnetic recording tape, an adhesive tape, pressure-sensitive recording paper, an offset printing plate material or the like.
A method of applying an electrostatic field to act has been disclosed along with a conventional method of coating to a continuously moving web by applying the electrostatic field to act on the web. For example, there has been a device disclosed in the Japanese Patent No. 27423/71 in which a backup roller is kept at a high potential to generate an electrostatic field between a coating hopper and the backup roller to exercise a method of applying the electrostatic field to act between the coating hopper and the backup roller or between the coating hopper and a web. A device for subjecting a web to a corona discharge treatment to cause an electrostatic field to act between a coating hopper and the web was disclosed in the published Japanese Patent Application (OPI) No. 167750/82 (the term "OPI" as used herein means an "unexamined published application").
Such a method of applying an electrostatic field to act produces an effect which prevents an air film often made on a rapidly moved web in coating to the web from hindering the liquid from wetting the web. However, there has not been a patent application in which the quality and structure of the surface of a web are descibed with regard to a method of coating to the web by applying an electrostatic field to act thereto.
In order to increase the speed of coating of a liquid to a web without increasing the load of drying the coated layers, it is necessary to increase the concentration of a solute in the coated layer, for example, the concentration of gelatin which is often used as the protective binder of a photographic photosensitive material. However, there is a problem that if the web is provided with an undercoating layer of gelatin to augment the adhesive power between the web and the coated layer, the speed of coating the web can hardly be increased by a method in which an electrostatic field is caused to act to the web to coat the liquid thereto.